Invasive fungal infections are a significant cause of morbidity and mortality in severely ill patients, and their impact is exacerbated by a failure to rapidly diagnose and effectively treat these infections. The widespread use of antifungal agents has resulted in selection of naturally resistant fungal species, as well as the emergence of resistance in susceptible species. Recently, the echinocandin drugs, caspofungin and micafungin, were introduced as a new class of highly active antifungal agents that target the fungal cell wall by blocking [unreadable]-(1-->3)-D-glucan synthase. Echinocandin use is growing rapidly and clinical isolates of Candida with reduced in vitro susceptibility are being reported. Treatment of fungal disease is hampered by the availability of limited drug classes with some treatment failures correlating with high in vitro MIC values. It is anticipated that the number of clinical isolates with elevated MIC values will rise as patient exposure to echinocandin drugs broadens, and as newer compounds such anidulafungin enter the market. As the echinocandins are the first new major antifungal drug class in decades, it is vital to understand the nature of developing resistance mechanisms to this class of drugs. Recently, we reported that mutations in two regions of Fks1, a principal component of the glucan synthase complex, account for resistance in laboratory and clinical isolates of Candida albicans and other Candida spp. The objective of this proposal is to explore in detail by genetic and biochemical means the role of Fks1 as an important new mechanism for clinical resistance to echinocandin drugs. The specific aims are designed to: 1) provide a detailed molecular description of regions of Fks1 associated with resistance to echinocandin drugs and to define the physical limits of these regions and ranges of mutations that contribute to resistance;2) define structural properties of Fks1-drug interactions and assess whether all echinocandin drugs bind to glucan synthase at the same site, and whether a single mechanism of resistance is sufficient for all echinocandin drugs;3) develop a membrane topology model of Fks1 that can be used to better understand functional domains, and 4) evaluate in clinical isolates the frequency of the Fks1 resistance mechanism. The overall objective of this program is to develop a comprehensive molecular description of Fks1- mediated resistance to echinocandin drugs that relates to clinical efficacy.